The vertical tail plane of an aircraft and the vertical rudder mounted to the vertical tail plane provide a means to control a yawing moment for rotating the aircraft about its vertical or yawing axis. They therefore provide directional stability and control. The maximum yawing moment that can be generated by the rudder depends on the size of the rudder and the degree by which the rudder can be deflected about a rudder axis. In other words, more yawing moment can be generated with a larger rudder or a rudder that can be deflected by a greater angle. As the rudder can only be deflected by a limited angle to avoid stalling of the flow at the rudder, the maximum yawing moment is largely determined by the overall shape and surface area of the rudder and the vertical tail plane.
Maximum yawing moment is never required under regular operating conditions, therefore most of the yawing moment that could be generated by the vertical rudder remains unused, which results in unnecessary aerodynamic drag and as drag increases, the fuel consumption increases, which reduces the maximum range of the aircraft. Therefore, any unnecessary drag should be avoided.